1. Field of the Invention
The present invention relates to a multi-voltage level power supply unit and a switching circuit employed in the power supply unit.
2. Description of the Related Art
Vehicles such as automobiles have a power supply unit for supplying loads with currents. Such a power supply unit supplies a load with the output voltage of a 14-volt power supply via the drain electrode D and the source electrode S of a field effect transistor (FET).
One of the known power supply unites is a multi-voltage level power supply unit shown in FIG. 1. As shown in FIG. 1, an AC voltage generated by an alternator 121 is converted by an AC/DC converter 122 into a DC voltage, which is then transferred to a 42-volt power supply B1. The output of the 42-volt power supply B1 is supplied to a load 123 such as a driving motor. The DC voltage output of the 42-volt power supply B1 is lowered by a DC/DC converter 125 and transferred to a 14-volt power supply B2 of which the output is supplied to a load 127 such as a lamp. By the multi-voltage levels provided with the 42-volt power supply B1 and the 14-volt power supply B2, both the 42-volt system load 123 such as a driving motor and the 14-volt system load 127 such as a lamp can be driven.
The voltage output of the 42-volt power supply B1 is substantially three times greater than that of the 14-volt power supply B2 and its current is as small as ⅓. This permits the wire harness (W/H) in the 42-volt system circuit to be substantially ⅓ smaller in the diameter than that in the 14-volt system circuit. Accordingly, the W/H can be reduced in the weight and its load efficiency will be improved. For that reason, it is increasingly desired to activate a number of loads with the 42-volt power supply B1 in every vehicle. In case that the motor is driven by the 42-volt power supply B1 through a relevant relay, the use of a 42-volt system relay may increase the cost. It is hence desired to drive the motor with the 42-volt power supply B1, but using a 14-volt system relay.
However, when the 14-volt system relay is connected directly with the 42-volt power supply B1, it receives a too-high voltage and may generate an arc discharge (a spark) upon opening or closing, thus causing unwanted worn-out or meltdown on the contact.
For preventing the worn-out or meltdown of the contact so as to extend the life of the relay, an ON/OFF control circuit employing a semiconductor device and a relay is proposed in Japanese Patent Laid-open Publication S59-221921. The ON/OFF control circuit has, as shown in FIG. 2, a driver 205, an AND circuit 209, the semiconductor device 206 and the relay 207 connected in series with each other, and a timing circuit 208 for controlling the timing of switching on and off the semiconductor device 206 as well as the timing of turning on and off the relay 207. The relay 207 is activated for turning on or off after the semiconductor device 206 is changed to the off state by the timing circuit 208. In this circuit, the semiconductor device 206 is driven to off state before turning on and off the relay 207, so that a flow of large current through the contact of the relay 207 can be suppressed. Accordingly, the life of tile relay 207 can be extended.
For suppressing the arc discharge in the relay so as to prevent the worn-out or meltdown on the contact, a switching apparatus is proposed in Japanese Patent Laid-open Publication S56-116238. The switching apparatus has, as shown in FIG. 3, an electromagnetic relay 302 having normally open contacts 302a, 302b, 302c and a triac 305 connected in series with the normally open contact 302b. The switching apparatus further has a normally-open push-button switch 303 and a normally-closed push-button switch 304 connected in series with the electromagnetic relay 302. It is so set that, when the normally-open push-button switch is closed to flow current in the electromagnetic relay 302, the normally open contact 302b closes first, and the normally open contact 302c closes so as to conduct the triac 305. On the contrary, the normally open contact 302c opens first to cut off the triac 305, and the normally open contact 302b opens afterwards, when the normally-closed push-button switch 304 becomes open so as to cut off the current in the electromagnetic relay 302, thus minimizing melting-down or worn-out of the contact 302b. 
It is therefore an object of the present invention to provide a switching circuit in which a reverse electromotive force possibly generated on a load, which will be connected to a relay serving as a component of the switching circuit, can be minimized, even if the load is an inductive load such as a motor.
Another object of the present invention is to provide a switching circuit in which a voltage applied to the relay for closing and opening operations is effectively lowered so as to allow a configuration of a multi-voltage level power supply unit, in which higher and lower voltages are supplied.
Still another object of the present invention is to provide a switching circuit in which generation of arc discharge on the relay designed to operate at lower rated voltage can be minimized even in the multi-voltage level power supply unit.
Still another object of the present invention is to provide a switching circuit in which the relay designed to operate at lower rated voltage can be operated with higher supply voltage without using any specific and complicated mechanism or apparatus designed to operate at the higher voltage.
Still another object of the present invention is to provide a multi-voltage level power supply unit in which the relay, designed to operate at lower rated voltage, can be operated with the higher voltage without using a specific mechanism or apparatus by suppressing the generation of arc discharge.
Still another object of the present invention is to provide a multi-voltage level power supply unit in which the relay, designed to operate at the lower rated voltage, can be operated with the higher voltage without giving any damage, fracture, or breakdown to the semiconductor device, which may be employed in the unit, when it is switched off.
For accomplishing the foregoing objects, the present invention is implemented by the following manners. A first aspect of the present invention inheres in a switching circuit for use in a multi-voltage level power supply unit for supplying a first voltage and a second voltage lower than the first voltage. Namely, the switching circuit of the present invention has a first relay designed to operate at the second voltage, a first semiconductor device and a control unit. The first semiconductor device has a first control electrode, a first main electrode for receiving the first voltage, a second main electrode connected to the first relay. And the control unit is connected to the first control electrode of the first semiconductor device. The control unit provides a first control signal to the first control electrode so as to increase an interelectrode voltage between the first and second main electrodes, only during transition periods between open to closed states and closed to open states. Further the control unit provides second control signal to the first control electrode so as to decrease the interelectrode voltage during steady state periods of the first relay. The first semiconductor device of the present invention is always operating in its on state, but the operation point of the first semiconductor device swings between the higher and lower interelectrode voltages.
In the first aspect of the present invention, the first semiconductor device may be a bipolar transistor (BJT), a field effect transistor (FET), a static induction transistor (SIT), insulated gate bipolar transistor (IGBT), etc. For example, if the first semiconductor device is the BJT, the first control electrode is a base electrode, the first main electrode may be emitter or collector electrode. And the second main electrode is collector or emitter electrode other than the first main electrode. Then the interelectrode voltage between the first and second main electrodes is the emitter-collector voltage. If the first semiconductor device is the FET, the first control electrode is a gate electrode, the first main electrode may be source or drain electrode. And the second main electrode is drain or source electrode opposing to the first main electrode. Then the interelectrode voltage between the first and second main electrodes is the source-drain voltage.
In the switching circuit employed in a vehicle, the first voltage may be 42-volts and the second voltage may be 14-volts, for example. However, the switching circuit of the present invention is not limited to a 42-volt/14-volt power supply but may be at any other rates.
According to the first aspect, when the first relay is scheduled to be closed, the control unit increases the interelectrode voltage to a level greater than the interelectrode voltage with the first relay remaining closed in the steady state period, and then closes the first relay. And, when the first relay is scheduled to be open, the control unit increases interelectrode voltage to a level greater than the interelectrode voltage with the first relay remaining open in the steady state period and then opens the first relay. This allows the voltage to be applied to the first relay to be significantly lowered to a level smaller than the first voltage, before the first relay is driven to opened and closed states.
Accordingly, the reverse electromotive force possibly generated on a load, which will be connected to the first relay, can be minimized, even if the load is an inductive load such as a motor, hence preventing fracture, breakdown, or damage of the first semiconductor device. Also, as the voltage applied to the first relay for triggering the closing and opening operations is effectively lowered, the generation of arc discharge on the first relay designed to operate at the second voltage can be minimized. Further, the first relay designed to operate at the second voltage can be operated with the first power supply without using any specific and complicated mechanism designed to operate at the first voltage.
A second aspect of the present invention inheres in a multi-voltage level power supply unit having a constant voltage source for providing a first voltage, a switching circuit connected to the constant voltage source and a DC/DC converter for reducing the first voltage so as to provide the second voltage. The first voltage may be 42-volts and the second voltage may be 14-volts, for example. Here the switching circuit is connected to the constant voltage source supplying the first voltage, or the higher voltage. And the switching circuit has a first relay designed to operate at the second voltage, a first semiconductor device and a control unit, similarly to the first aspect of the present invention. Namely the first semiconductor device has a first control electrode, a first main electrode for receiving the first voltage, a second main electrode connected to the first relay. And the control unit is connected to the first control electrode. The control unit provides a first control signal to the first control electrode so as to increase an interelectrode voltage between the first and second main electrodes, only during transition periods between open to closed states and closed to open states. Further the control unit provides second control signal to the first control electrode so as to decrease the interelectrode voltage during steady state periods of the first relay.
According to the second aspect of the present invention, when the first relay is scheduled to be closed, the control unit increases the interelectrode voltage to a level greater than the interelectrode voltage with the first relay remaining closed and then closes the first relay. And, when the first relay is scheduled to be open, the control unit increases the interelectrode voltage to a level greater than the interelectrode voltage with the first relay remaining open and then opens the first relay. This allows the voltage to be applied to the first relay to be significantly lowered to a level smaller than the first voltage, before the first relay is driven to opened and closed states. Accordingly the reverse electromotive force possibly generated on a load, which will be connected to the first relay, can be minimized, even if the load is an inductive load such as a motor, hence preventing fracture, breakdown, or damages of the semiconductor device. Also, as the voltage applied to the first relay for triggering the closing and opening operations is effectively lowered, the generation of arc discharge on the first relay designed to operate at the second voltage can be minimized. Simultaneously, the first relay designed to operate at the second voltage can be operated with the first power supply without using any specific mechanism designed to operate at the first voltage.